Cairo is the capital of Egypt and one of the largest cities in Africa. Cairo is the gateway to the Nile delta, where the lower Nile separates into the Rosetta and Damietta branches. Cairo is a place of physical contrast. Cairo juxtaposes ancient and new, East and West. The Pyramids of Giza, near Memphis, stand at the southwestern edge of the metropolis, and an obelisk in the northeast marks the site of Heliopolis, where Plato once studied; modern landmarks of Western-style high-rise hotels and apartment buildings overlook the Nile River. Cairo contains a large functioning bazaar and an extensive, semi-walled medieval city endowed with more than 400 registered historic monuments—including mosques, mausoleums, and massive stone gates—dating to 130 CE. Cairo is fan-shaped, narrowest in the south, where the river valley is wedged between desert escarpments, and widest in the north, where the valley blends into the delta.